The present invention relates to a frequency stabilized waveguide arrangement for microwaves and the like.
Waveguides and cavity resonators which are formed as waveguides with reflecting end walls or screens are frequently used in microwave technology, for example as filters. The resonance frequency of such cavity resonators depends on the dimensions, in particular the axial length of the resonator. Since the waveguide material thermally expands with raising temperatures, the resonance frequency of a resonator reduces with increasing temperature. A temperature increase on the other hand can not be avoided with high power components due to the energy dissipation.
It is known to produce waveguides of a material with low temperature expansion coefficients, such as Invar or Superinvar. Invar has a thermal expansion coefficient of approximately 1.5 ppm/K. This material however has the disadvantage that the thermal conductivity is poor and dissipated heat can be withdrawn only insufficiently, so that the waveguide arrangement is further heated. Aluminum is preferable as a material with high thermal conductivity and in addition low weight, in particular for environmental uses. However, on the other hand it has a high temperature expansion coefficient in the region of 22-24 ppm/K.
International published patent application WO 87/03745 and European patent EP 0 621 651 B1 disclose temperature-compensated cavity resonators which have curved screens facing toward the interior of the resonator and having a curvature which increases with increasing temperature. Thereby a temperature-dependent longitudinal expansion of the cavity resonator is compensated. The curved screens, for example end walls, are however expensive and costly in production, and must be individually dimensioned with regard to the frequency conditions.
When for example a filter composed of several cylindrical Invar resonators is held by aluminum mounting elements, the thermally dependent deformations occur at the contact points of the different materials. When an aluminum mounting element, for example engages an Invar flange of the resonator, a temperature-dependent bending of the resonator screen and thereby an additional undesirable frequency shift occurs.
A similar problem arises when a waveguide of Invar is coupled with a further waveguide of another material such as aluminum, which has a higher thermal expansion coefficient. In this case, due to the different thermal expansion coefficients of the materials, the both coupled waveguides are subjected to thermally-dependent deformations which lead to a frequency shift.
Accordingly, it is an object of the present invention to provide a frequency-stabilized waveguide arrangement, which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a waveguide arrangement which has a first waveguide composed of a material with a first thermal expansion coefficient, a second waveguide composed of a material with a second thermal expansion coefficient, and a transition element provided between the first waveguide and the second waveguide for mechanical uncoupling of the different thermal expansion coefficients of the both waveguides.
The inventive waveguide arrangement has the disadvantage that despite different materials of the both waveguides, a thermally-dependent deformation and frequency shift is minimized. At least one of the both waveguides can be a resonator with end walls, for example screens. Due to the transition element, a deformation of the end walls or screens is also minimized.
A transition element can be for example a circular, outwardly open gap. This permits a bending at the border surface of the two waveguides.
The transition element can be provided with milled depression which allows a deformation of the screen mounted on it or adjoining it, in both directions.
In accordance with another embodiment of the invention, the waveguide arrangement has a mounting element applied on a flange of the waveguide. It has a material with a different thermal expansion coefficient than the waveguides, or its screens and associated flanges. On the flange or the end wall/screen, a compensation element can be provided for compensation of thermal deformation of the end wall which is caused by different thermal expansion coefficients. Preferably, the mounting element can be composed of aluminum and the compensation element can be composed of Invar.
This inventive waveguide arrangement has the advantage that, for mechanical holding of the waveguide arrangement and for withdrawing of heat, the mounting elements of a material such as aluminum can be used, which has a high thermal conductivity, but also a high thermal expansion coefficient, without causing an additional thermal-dependent frequency detuning.
The inventive waveguide arrangement in accordance with another embodiment has a ring-shaped compensation for compensation of thermal deformation of the waveguide, and the compensation means have a different thermal expansion coefficient than the waveguides. The compensation means can have a higher thermal expansion coefficient than the waveguides, so that with increasing temperature for compensation an axial expansion of the resonator, the end wall or the screen of the waveguide is deformed inwardly. Therefore a thermal expansion of the waveguide can be compensated.
The end wall or the screen of the waveguide arrangement can be for example flat at ambient temperature. When compared with an initial condition at ambient temperature of curved end walls or screens, there is a disadvantage of an easier and therefore less expensive manufacture.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.